DIFFUSE-REFLECTANCE SPECTROSCOPY STUDY OF THE THERMAL GENESIS AND MOLECULAR-STRUCTURE OF CHROMIUM-SUPPORTED CATALYSTS

The diffuse reflectance spectra of a series of chromium-supported sili ca aluminas, with varying SiO2 content, have been investigated before and after drying and after calcination, successive CO reductions, and recalcination. The molecular structure of chromium before drying is in fluenced by the isoelectric point (IEP) of the supports. It is conclud ed that the lower the IEP of the support, the higher the amount of sur face dichromate. After calcination at 720 degrees C, Cr is anchored to the surface without changing the chromate:dichromate ratio. Reduction results in the formation of octahedral Cr3+, pseudooctahedral and pse udotetrahedral Cr2+. A higher silica content of the support, a higher reduction temperature, and a higher calcination temperature result in a deeper reduction of the supported Cr. Recalcination restores the ini tial chromate:dichromate ratio on each support; however, small quantit ies of Cr3+ remain in the samples. This deeper reduction and reversibi lity can be explained by the low OH content of the supports calcined a t 720 degrees C.